Rotary cutting dies are widely used for manufacturing boxes, cartons and interpak die cuts. The rotary die includes counter-rotating die and anvil rolls mounted in parallel spaced relation to one another. Both the die and anvil rolls typically comprise a metal cylinder. A cutting die is mounted on the die roll and includes a series of cutting rules and creasing rules. The anvil roll is covered with urethane or other resilient material. When the sheet material is fed between the rolls, the cutting die in cooperation with the anvil roll, cuts out a blank from the sheet material and forms creases where the blank is to be folded.
The cutting die includes a die board having a plurality of slits into which a series of cutting rules and creasing rules are fitted. In fabricating the cutting die, a pattern is laid out on a die board which corresponds to the shape of the blank being formed. Saw cuts are made in the die board to form the slots for holding the cutting and creasing rules. The cutting and creasing rules are inserted into the slots thus formed and project outwardly from the surface of the die board to engage the corrugated paper board as it is fed between the rolls.
The cutting rule i made from a flat piece of steel and has a serrated edge for cutting the corrugated paper board. The cutting rule is usually for four-point thickness (a point is equal to 0.01384 inches or approximately 1/72 of an inch). The creasing rule is also made from a flat piece of steel but has a round edge, instead of a serrated edge, for compressing the board to form a crease or fold line. Further, the creasing rule may have a thickness of two points, three points, four points, six points, eight points or more depending on the gauge of the corrugated stock to be creased. Generally, the gauge of the stock to be creased will dictate the size of the creasing rule. As the gauge of the stock increases, a thicker creasing rule is needed to satisfy the fold relief of the corrugated stock.
In the past, the thickness of the creasing rule was uniform throughout the cross section of the rule. Creasing rules of uniform thickness, however, were found to have significant drawbacks. For example, when placing a cutting rule of four point thickness end-to-end with a creasing rule of different thickness, there is some difficulty in lining the creasing rule with the cutting rule. To maintain the creasing rule and cutting rule in line, saw cuts of different widths must be made on the die board. To keep the rules in line, these saw cuts must be carefully placed, thus increasing the time and expense in the manufacture of the cutting die.
Creasing rules of uniform thickness also make it difficult to sue the same die on different thicknesses of stock. For example, if the size of the stock was changed so that a different size creasing rule was required, the slots formed in the die board would not accommodate the different thickness or width rule. Thus, a separate cutting die would have to be manufactured which was identical to the first die, but with slots formed to receive the larger creasing rules.
One solution to these problems is to vary the thickness of the creasing rule throughout its cross section such that the base portion (the portion which fits into the slots of the die plate) would be made the same thickness as the cutting rule. The top portion could be made of a larger thickness, such as four points or six points. Since the base portion is uniform on all of the creasing rules, different size creasing rules can be used interchangeably in the same cutting die. However, the creasing rules with non-uniform thickness are more costly to manufacture. Manufacture of these creasing rules is usually done by welding a thicker piece of metal stock to a conventional four point rule. This process is both expensive and time consuming.
Another undesirable result associated with the use of prior art creasing rules is that the creasing rules sometimes "crack" the surface layer of the corrugated paper board, particularly when the fold lines are made in a direction parallel to the flutes of the corrugated paper board. Once the surface layer of the corrugated paper board is "cracked", there is tendency for the crack to run the entire length of the board. The presence of "cracked" scores is undesirable and may render the boxes or cartons unusable.